Crucifixion practices

A quick survey of references must begin with Herodotus (484?-425 BCE), who in his History records a number of impalements amongst the ancient Persians:

But no sooner had Polycrates come to Magnesia than he was horribly murdered in a way unworthy of him and of his aims; for, except for the sovereigns of Syracuse, no sovereign of Greek race is fit to be compared with Polycrates for magnificence. Having killed him in some way not fit to be told, Oroetes then crucified him…
Herodotus, Histories, 3.125.3, Perseus Text Archive

Notice the use of crucifixion as an exhibition of the dead, a recurring theme. Other references include Darius’ impalement of 3,000 men after capturing Babylon. It is not always possible to delineate between impalement on a stake, and crucifixion on a pole (with or without a cross-beam), since the same terms were used interchangeably over time (eg. in Greek, stavros, even when a cross bar is envisaged, as in the gospels), and because there was no ‘standard’ crucifixion -- it would be fair to conjecture that almost every concievable method of hanging somebody up to die in public was attempted at one time or another. Seneca, whose life overlapped that of Jesus, recorded that even in his day, impalement and crucifixion were common variants opon the same theme.

Yonder I see instruments of torture, not indeed of a single kind, but differently contrived by different peoples; some hang their victims with head toward the ground, some impale their private parts, others stretch out their arms on a fork-shaped gibbet; I see cords, I see scourges, and for each separate limb and each joint there is a separate engine of torture!
Seneca, Letter of Consolation to Marciam, 20 (Loeb Edition).

Other ancient civilizations connected with the practice include the Assyrians, Germanic and Britonic tribes, the Celts, the Greeks after Alexander the Great, the Phoenicians and the Carthaginians, from whom the Romans are believed to have taken the practice.

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Seneca (~5 BCE - 65 CE) provides a description of crucifixion in the course of arguing (Stoic that he was) that suicide is better than some forms of death.

Can anyone be found who would prefer wasting away in pain dying limb by limb, or letting out his life drop by drop, rather than expiring once for all? Can any man be found willing to be fastened to the accursed tree, long sickly, already deformed, swelling with ugly weals on shoulders and chest, and drawing the breath of life amid long-drawn-out agony? He would have many excuses for dying even before mounting the cross.
Seneca, Epistles (to Lucilius), 101.

The ‘ugly weals’ would be a reference to the standard practice of beating or flogging which preceded a crucifxion. 

An early example of crucifixions in Israel is associated with Antiochus IV (of Greece), after he had instituted pig sacrifices in the temple at Jerusalem and throughout Israel (c.169 BCE):

…many Jews there were who complied with the king's commands, either voluntarily, or out of fear of the penalty that was denounced. But the best men, and those of the noblest souls, did not regard him, but did pay a greater respect to the customs of their country than concern as to the punishment which he threatened to the disobedient; on which account they every day underwent great miseries and bitter torments; for they were whipped with rods, and their bodies were torn to pieces, and were crucified, while they were still alive, and breathed. They also strangled those women and their sons whom they had circumcised, as the king had appointed, hanging their sons about their necks as they were upon the crosses.
Josephus, Antiquities of the Jews, 12.248-256, Perseus Text Archive

Another example, a century later, is found when when Alexander (son of Cleopatra) invades Judaea:

the Jews fought against Alexander, and being beaten, were slain in great numbers in the several battles which they had; and when he had shut up the most powerful of them in the city Bethome, he besieged them therein; and when he had taken the city, and gotten the men into his power, he brought them to Jerusalem, and did one of the most barbarous actions in the world to them; for as he was feasting with his concubines, in the sight of all the city, he ordered about eight hundred of them to be crucified; and while they were living, he ordered the throats of their children and wives to be cut before their eyes.
Flavius Josephus, Antiquities of the Jews (ed. William Whiston, A.M.), 13.379-383, Perseus Text Archive

For the times closer to Jesus’ life, Josephus’ Jewish War (the normal translation of the title) is the primary source. In the quelling of a single civil disturbance, Josephus records:

…Varus sent a part of his army into the country, against those that had been the authors of this commotion, and as they caught great numbers of them, those that appeared to have been the least concerned in these tumults he put into custody, but such as were the most guilty he crucified; these were in number about two thousand.
Flavius Josephus, The Wars of the Jews (ed. William Whiston, A.M.), 2.66, Perseus Text Archive

There are numerous other examples in Josephus’ works of the very routine nature of crucifixion. In an interesting aside on Jewish burial customs, while describing the attacks opon Jerusalem by the Idumeans, he declares:

…they proceeded to that degree of impiety, as to cast away their dead bodies without burial, although the Jews used to take so much care of the burial of men, that they took down those that were condemned and crucified, and buried them before the going down of the sun.
Flavius Josephus, The Wars of the Jews (ed. William Whiston, A.M.), 4.305, Perseus Text Archive

Compare Deut.21:23 for the basis of this. Finally, as the seige of Jerusalem was nearing an end in 70 CE, we read of the many Jews in the city:

…the severity of the famine made them bold in thus going out; so nothing remained but that, when they were concealed from the robbers, they should be taken by the enemy; and when they were going to be taken, they were forced to defend themselves for fear of being punished; as after they had fought, they thought it too late to make any supplications for mercy; so they were first whipped, and then tormented with all sorts of tortures, before they died, and were then crucified before the wall of the city. This miserable procedure made Titus greatly to pity them, while they caught every day five hundred Jews; nay, some days they caught more: yet it did not appear to be safe for him to let those that were taken by force go their way, and to set a guard over so many he saw would be to make such as great deal them useless to him. The main reason why he did not forbid that cruelty was this, that he hoped the Jews might perhaps yield at that sight, out of fear lest they might themselves afterwards be liable to the same cruel treatment. So the soldiers, out of the wrath and hatred they bore the Jews, nailed those they caught, one after one way, and another after another, to the crosses, by way of jest, when their multitude was so great, that room was wanting for the crosses, and crosses wanting for the bodies.
Flavius Josephus, The Wars of the Jews (ed. William Whiston, A.M.), 4.305, Perseus Text Archive